Fix: Resolve all lint errors from ocr/ restructure

- Added ocr_region import to cell_grid/build.py and legacy.py
- Fixed circular import in engines.py via lazy import
- Auto-fixed 22 unused imports via ruff --fix

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

klausur-service/backend/ocr/layout/layout.py

@@ -17,7 +17,7 @@ Sub-modules:
 """
 
 import logging
-from typing import Any, Dict, List, Optional, Tuple
+from typing import Dict, List, Optional, Tuple
 
 import numpy as np